A Guide to
Responsible
External
Parasite Control
in Sheep
Contents
Introduction to Guidelines – Dr Peter Bates, VLA 3
Responsible External Parasite Control – Peter Morris, NSA
4
Ectoparasite Control Decision Tree
5
Blowfly Strike
6
Ticks
10
Chewing Lice
12
Sheep Scab
14
External Sheep Parasites:
Guidelines for Responsible Control
This guide is a practical introduction to the external parasites most
likely to jeopardise the welfare and productivity of your sheep:
scab, chewing lice, ticks and blowfly strike.
It aims to encourage better identification, and understanding,
of the parasites involved – their lifecycles, symptoms, the time of
year at which they occur – and the use of the most responsible
methods of control.
By preventing the misuse of chemical products, these guidelines
promote a long-term approach to sheep welfare designed to minimize
the build up of resistance and protect the future health
and productivity of flocks throughout the UK.
Dr. Peter Bates
Senior Entomologist – VLA
3
The Importance of Responsible External
Parasite Control
As individual sheep farmers, it is vital to give thought and planning
to decisions relating to ectoparasite control. The UK flock is under
severe pressure from a variety of ectoparasites and the range of
preventative measures and treatments at our disposal is, and will
continue to be, limited.
Some of those products – particularly the plunge dipping chemicals
– continue to be closely scrutinised and to ensure their continued
availability it is vital that they are used safely, responsibly and in line
with the recommendations of the ‘Stop every Drop’ campaign.
Using the right kind of product for the problem to be dealt with is
a key decision for all sheep farmers. If we do not make strategic
and considered choices, based on understanding the ectoparasite
challenges facing the flock at any given time of year, then the
emerging resistance issues that are becoming obvious in some
areas will continue to grow and place the sheep industry in an
impossible position.
Using ectoparasite controls strategically, in conjunction with health
planning, will not only provide the most effective control of parasites,
it will also ensure the longevity of the products and save you money.
NSA encourages all UK sheep farmers to adopt the principles
outlined in this guide to give the best possible long-term protection to
the national flock from ectoparasite infestations.
Peter Morris
Chief Executive, National Sheep Association
January 2007
4
Ectoparasite Control Decision Tree
Follow this decision tree to identify the ectoparasites affecting your
flock and select the most suitable methods of treatment.
What is the problem?
Itchy Sheep
Veterinary diagnosis
Blowfly
Ticks
Prevention
Treatment
Insect Growth
Regulator
(IGR)
SP Pour-on/
Spot-on
or OP Dip
Lice
SP Pour-on/
Spot-on
or OP Dip
Scab
OP Dip
or Injection
5
Blowfly Strike
What is it?
• Larvae (maggots) of three species of fly commonly
attack sheep in the UK: Lucilia sericata (the
“greenbottle”), Phormia terrae-novae (the “black
blowfly”) and Calliphora erythrocephala (the
“bluebottle”). These species are not totally parasitic
and large numbers are associated with the environmentally
useful tasks of faecal and carcass decomposition.
• Strike can be classified as wound (usually feet), body or breech.
In body strike, flies are attracted to sheep by the odours of
excessive “sweating” and/or decaying organic matter in the
fleece, usually over the loins, shoulders, flanks, neck, back,
throat or abdomen. In breech or tail strike, flies are attracted
to fleece contaminated with urine and/or faeces and are
particularly associated with scouring.
Symptoms
• Signs of strike include agitation and dejection, foot stamping,
vigorous shaking, gnawing or rubbing of the tail and breech.
As infestation develops a distinctive odour is noticeable and the
wool becomes matted and discoloured. If the infestation remains
untreated the affected area increases and wool is shed from the
centre, accompanied by signs of constant discomfort. Death will
occur soon after.
• The smell of infestation attracts further flies. If unchecked,
extensive infestations of further waves of flies can result in a
quick and agonizing death.
6
Timing
• The prevalence of blowfly strike is weather-dependent, with most
cases of body strike occurring during periods of high humidity or
warm periods after heavy rain. In the south east of England strikes
can occur any time between March and December. Breech strike
depends less on weather as the moisture supplied by urine and/or
scouring is sufficient for larval development.
• Blowflies are part of the normal flora and fauna of your farm – they
are there spring to autumn. Carcasses (small or large) and foot rot
can increase fly numbers. Make sure all carcasses are disposed of
and attend to infected feet as soon as possible.
7
Blowfly Strike
Blowfly Lifecycle
• Adult flies can lay up to 3,000 eggs in 9-10 batches over a
3 week period. Eggs hatch within 24 hours if the fleece humidity is
optimal. Eggs are continually being deposited on the fleece (even
in hot, dry weather), but first stage larvae cannot survive in wool
with a moisture content below 90 per cent. Later stages are less
dependent on humidity. Total development can take 5-11 days on a
carcass but only 3 days in body or breech strike or even shorter in
a wound. Larvae leave the sheep to pupate in the soil and remain
in the pupa for 3-21 days under summer conditions. Over-wintering
pupae remain inactive until the soil temperature rises above 70C.
The Blowfly lifecycle
Deposition of eggs in
the fleece
1st Larval Stage.
No mouthparts
Average life cycle
2-4 weeks
Fly emergence
after 2 weeks
warm weather
2nd and 3rd Larval Stages.
Mouthparts very active and
feed constantly.
Pupation
8
Prevention
• Evidence points to the possibility that some attraction factors are
hereditary and breeding ewes and rams continually struck could
be culled. Ewes with deformed genital openings, where urine
is directed onto the fleece and ewes with narrow breeches that
favour soiling could also be culled.
• Complete shearing temporarily reduces the risks of strike, but the
susceptibility of the sheep increases as the fleece grows. Routine
crutching and dagging is effective if started in April and repeated
every four to six weeks. Tail docking may also reduce the incidence
of strike.
• Prevention is the best form of control. Strike can be prevented
through the use of pour-ons containing the insect growth regulators
(IGRs) cyromazine or dicyclanil, however these products will not
cure existing infestations.
Treatment of existing strike
• Cure and protection can be attained using a diazinon based
plunge dip or a synthetic pyrethroid (SP) based pour-on containing
alphacypermethrin or cypermethrin. Spot-on products containing
the SP deltamethrin will cure existing infestations but will not
protect against further attack.
9
Ticks
What are they?
• Ticks (Ixodes spp, Haemophysalis spp, Dermacentor
spp) can have significant effects on sheep welfare
and production, particularly through the transmission
of louping ill, lamb pyaemia (“cripples”) or tick borne
fever (TBF).
Symptoms
Timing
• Ticks are generally active from March until October, depending on
the weather.
10
Tick Lifecycle
• The tick life cycle includes egg, larva, nymph and adult (male or
female). The entire life cycle can take 1 to 5 years depending on
the weather and availability of suitable hosts.
• Ticks feed on blood – seeking suitable blood vessels in/on the
ears, groin and armpits of the sheep. Ticks feeding on sheep are
mainly adults. Adult ticks will also feed on cattle, horses, deer, etc.
Nymphs generally feed on smaller hosts (foxes, rabbits etc) and
larvae feed on even smaller hosts (birds, mice, etc).
• Each stage feeds only once, engorging on blood for 2 to 15 days
and then dropping off into the environment to digest its bloodmeal
and moult into the next stage.
• Ticks require dense moisture retentive habitats, eg. rough grazing,
moorland, heath or woodland. The thicker the vegetation the
heavier the tick population.
• Ticks are becoming more abundant due to changes in farming
practice, climate change and acaricide resistance.
• Adult ticks will also feed on humans where they can transmit Lyme
Disease – a serious and potentially debilitating disease.
Prevention
• Prevention is the best form of control. Ticks can be prevented
through the use of a diazinon based plunge dip or a synthetic
pyrethroid based pour-on containing alphacypermethrin or
cypermethrin or spot-on products containing the SP deltamethrin.
11
Chewing Lice
What are they?
• Chewing lice (Bovicola ovis) are small, pale
to red/brown, flat, insects, feeding on skin
debris and hair, living within the centimetre of
wool closest to the skin. They do not produce a
definite scab lesion and can be found anywhere
on the sheep.
Symptoms
• Infested sheep rub and scratch and the fleece appears rough and
pulled, with numerous loose strands of wool. Sheep can bite and
nibble at the fleece. There is no biting reflex on handling.
• Heavy infestations are associated with sheep in poor health. Lice
populations are influenced by body condition; the lower the body
condition score the higher the population of lice. Thus, chewing
lice are a significant indicator of underlying welfare problems within
a flock.
12
Timing
• Lice are generally seen between January and March.
Chewing Lice Lifecycle
• Lice live their entire lives on the sheep, introduced by contact
or purchase of infested stock. Lice can, however, live off the
sheep for 16 to 17 days in tags of wool, clothing, barns or
livestock transport.
Prevention
• Quarantine all incoming stock for at least 3 weeks. Ensure
fencing is sound – prevent straying on or off. Disinfect
livestock trailers after use. Remove all debris (wool etc) from
contaminated housing and do not re-stock for at least three
weeks.
Treatment
• Act as soon as you see the sheep rubbing and scratching.
Have the sheep examined by your vet – he/she will identify any
parasite and advise on treatment. The cost of not involving a vet
will be far more than the cost of veterinary inspection!
• Sheep can also have mixed infestations of sheep scab and
chewing lice.
• Chewing lice can be controlled by using a diazinon based dip
or synthetic pyrethroid (SP) based pour-ons or spot-ons using
alphacypermethrin, cypermethrin or deltamethrin.
13
Sheep Scab
What is it?
• Sheep scab is caused by a tiny, pearly-white mite
(Psoroptes ovis), just visible to the naked eye. Mites
live on the skin surface, causing a skin allergy to
their droppings. Initially the scab is very small (less
than the diameter of 1p coin). As the mites multiply
the scab spreads, eventually covering the whole sheep. Mites can
be found around the edge of the growing lesion.
Symptoms
• Early disease is difficult to spot – animals look and behave
normally. As the scab spreads the sheep becomes increasingly
irritated, excessively rubbing and scratching and digging with their
back legs. After about 10 to 15 weeks the scab covers the entire
animal and the wool can fall out, revealing a dry, scabby skin
beneath. By this time scab has spread throughout the flock making
control a costly exercise!
• Infested animals can demonstrate a violent biting reflex on
handling (similar to scrapie), sometimes leading to epileptiform fits.
14
Timing
• Scab can be found throughout the year, but the majority of cases
occur between October and March.
Sheep Scab Lifecycle
• Scab mites live their entire lives on the sheep, introduced into
a flock through contact or purchase of infested stock. Mites can
however live off the sheep for 16 to 17 days in tags of scabby
wool, clothing, barns or livestock transport.
Prevention
• Prevent by quarantining incoming stock for at least 3 weeks.
Ensure fencing is sound to prevent straying on or off. Disinfect
livestock trailers after use. Remove all debris (wool etc) from
contaminated housing and do not re-stock for at least three weeks.
Treatment
• Don’t wait for wool to fall out - act as soon as you see the sheep
rubbing and scratching. Ask your vet to examine the sheep to
identify any parasite and advise on treatment. Not involving a vet
will be far more costly than prompt veterinary inspection!
• The signs of scab can be confused with those of lice. Resistance
may occur in both ectoparasites if they are not professionally
identified and treated. Sheep can also have mixed infestations of
sheep scab and chewing lice.
• Sheep scab can be controlled by using a diazinon based dip or by
doramectin, ivermectin or moxidectin based injections. Remember
that injections are also effective wormers and misuse could lead
to drug resistant worms – consult your vet.
15
Sponsored by
For further information contact: Novartis Animal Health UK Ltd.,
New Cambridge House, Litlington,
Nr. Royston, Hertfordshire SG8 0SS Tel: 0800 801566
FAD 730 JAN 2007